Infrared imaging is the detection of infrared radiation to produce an image capable of being visualized by the human eye. Infrared imaging systems detect thermal radiance differences among objects in a scene, and display these differences as a visual image of the scene. Certain infrared radiation characteristics must be dealt with to obtain a satisfactory image. One such consideration is the background radiance from a scene, which is a large component of the total infrared radiation emitted by the scene. Radiance differences among the objects are small compared to this background radiance.
Infrared imaging systems, described at a broad level, include several basic components: optics for collecting and focusing infrared radiation from a source, an infrared detector for converting the radiance to an electrical signal, and electronics for amplifying and processing the signal for display or storage. These systems use a variety of infrared detectors, which can be grouped into two main categories: uncooled and cooled. Uncooled detectors include thermal detectors, whose temperature is changed by incident radiation. Cooled detectors include photoconductors, whose electrical conductivity is affected by incident radiation, or photovoltaic devices, which generate voltage as a result of incident radiation.
An infrared detector generates a signal in response to radiation emitted by a viewed scene. Either "staring" or scanning techniques may be used to produce the signal, but in either case, the signal has the same characteristics. The bias level of the signal represents the background radiation. The signal's modulation represents radiance differences, and is the part of the signal that differentiates objects within the scene. Yet, because of the large background radiance, the modulation is small compared to the signal's amplitude, making amplification difficult.
Accordingly, efforts have been made to isolate the modulated part of the signal. One approach is to determine a signal reference level, which is then subtracted from the overall signal to obtain a signal that contains only the modulation levels. One implementation of this approach is optical devices that artificially create a reference image An example of one such system uses thermal electric (PELTIER) coolers to produce an intermediate image, which then determines the reference. Another implementation uses a signal chopper to produce a defocused image, which represents the background radiance.
Disadvantages of these past efforts are the expense of cooling and defocusing components and the complication of the optics. The common problem has been to find an optimum way to produce a true reference signal that can be subtracted from the overall signal to produce a signal with minimum background bias.